Ocular aberrations typically produce unwanted results in the form of bad eyesight. Accurately characterizing these aberrations can lead to appropriate prescriptions and methods for treatment. Since typically 60-70% of ocular aberrations result from imperfections in the cornea, the ability to determine the corneal topography of an eye is highly desirable. Corneal topography is typically determined with a device called a corneal topographer. A variety of corneal topographers are known in the art, examples of which are disclosed in U.S. Pat. Nos. 5,062,702, 6,634,752, and 7,976,163, which are herein incorporated by reference.
One type of corneal topographer employs a “Placido disk” system. A Placido disk system consists of a series of concentric illuminated rings that are reflected off the cornea and viewed with a detector array, such as a charge-coupled device (CCD) or a video camera. Because of its simplicity, the Placido disk topography system has been widely used for measuring corneal topography. A key part of this system is the object or device surface with rings, as well as the spatial distribution and the width of these rings on the surface of the device. The location and width of the rings on the device are computed in such a way that the image of the rings reflected off a reference sphere is a uniform distribution of rings, i.e., rings equally spaced, and all with the same width. The radius of curvature of the reference sphere is made equal to the mean radius of the cornea (about 7.8 mm). Then, the image of the rings reflected off a cornea with aberrations will constitute of distorted rings, and from this distortion, one can obtain the shape of the cornea.
Many variations on the Placido disk approach for corneal topography measurements have been developed over the years, examples of which are disclosed in U.S. Pat. Nos. 4,993,826 and 6,601,956, and by Yobani Meji'a-Barbosa et al., “Object surface for applying a modified Hartmann test to measure corneal topography,” APPLIED OPTICS, Vol. 40, No. 31 (Nov. 1, 2001) (“Meji'a-Barbosa”). Meji'a-Barbosa is incorporated herein by reference for all purposes as if fully set forth herein.
One problem in many Placido disk type corneal topographers is alignment error, which is commonly referred to as a vertex error between the corneal surface vertex and the design corneal vertex plane. More specifically, to make accurate calculations of corneal topography, the device expects the cornea to be located at a particular location long the optical axis of the system with respect to the Placido light sources. If an actual cornea that is being measured is “too close” or “too far” from the instrument or device, vertex error that will produce inaccurate corneal topography results, unless the vertex error can be determined and factored into the corneal topography calculations.
Another problem with Placido disk type corneal topographers is that the data is obtained from analysis of a series of projected rings. In other words, a radial position of the detected ring is compared to a reference position and the comparison is used to determine the corneal shape. This, however, only provides radial deviations. While these are azimuthally resolved, they do not provide an adequate measure of the “skew” rays, i.e., those rays which would be deflected in an azimuthal direction. This is an inherent limitation for a system using Placido rings topographers. The limitation is especially significant considering that astigmatism, one of the major classes of ocular aberrations, is known to generate significant skew rays.
Instead of using concentric rings, other corneal topographers have been developed that employ a light pattern comprising an array of light sources provided on a surface having the shape of a conical frustrum, a hemisphere or other modified sphere, or an elongated oval and the like. This light pattern is projected onto the cornea of the eye, and corneal topography is determined by observing the reflected light pattern of reflected spots from the cornea, and comparing this pattern to the projected light pattern from the light sources. In such a system and method of corneal topography, it is important to match each reflected light spot in the reflected light pattern to the projected light source which produced it so as to make an accurate comparison. This can be difficult for corneas with highly aberrated topographies. But, such matching may be improved if the pattern of projected light sources could be reconfigured dynamically to create easily recognizable fiducials, and/or to increase the density of the reflected light sources in areas which map to more highly aberrated portions of the cornea.
Unfortunately, in many known corneal topographers that employ a pattern of projected light sources, the pattern cannot easily be reconfigured to change the colors, positions, shapes, sizes, and localized densities of the projected light spots in the pattern.
Furthermore, whether they employ Placido disks or an array of light sources, these known conical topographers employ relatively complex light generating structures, which typically do not easily lend themselves to small, portable, and relatively inexpensive corneal topography constructions that might be desirable for providing corneal topography service to poor, remote, and rural populations.